The invention relates to variable frequency transformers (VFTS) and, particularly, to the arrangement of rotary transformers, controls and infrastructure in VFTs.
Variable frequency transformers are conventionally used to connect one power grid to another power grid. The two power grids carry alternating electrical current (AC) at potentially different frequencies, phases and voltages. A VFT provides a controlled alternating current (AC) path between two power grids. A VFT converts the AC power from one power grid to a frequency, phase and voltage that is suitable for the AC power in the other power grid.
A variable frequency transformer is a continuously phase shifting transformer that operates at an adjustable phase angle. The VFT may be a bi-directional current transmission device that converts AC current from a first power grid for a second power grid, and vice versa. A VFT typically includes a rotary transformer with three-phase windings on the rotor and the stator. The collector for the rotary transformer conducts current between the three-phase rotor winding and stationary conductors, e.g., busses, associated with the VFT. A drive motor and drive system adjusts a rotational position of a rotor in the VFT relative to the VFT stator to control the magnitude, frequency, phase and direction of AC power flowing through the VFT.
The rotary transformer in a VFT is conventionally a single large transformer such as used in hydro-power generation. These transformers rotate in a horizontal plane about a large vertical shaft. While conventional variable frequency transformers have been arranged in parallel at a substation to form a large asynchronous interconnection between power grids, these parallel transformers were operated and controlled as separate transformers. The transformers are massive and the shafts for the transformers are extensive. Because of the large transformer with a long vertical shaft, the VFT requires a special building with a high ceiling.
VFTs are conventionally large devices that are relatively tall, in that they typically include a large rotary transformer having a vertical rotating shaft. Conventional VFTs typically require expensive infrastructure to support the transformer and are housed in special tall buildings. A VFT may be installed at a substation between two electrical grids. These substations are often remote and not easily accessible for construction of the VFT. The construction costs for installing a VFT at a remote substation can be substantial.